Editorial: Mass and Angular Momentum Transport of Rapidly Rotating Hot Stars

This dedicated journal collection will present and discuss a variety of science cases that can be used to extend our knowledge of massive stars and the influence of their rapid rotation on their subsequent evolution. The aim is to build understanding of this pivotal class of stellar objects that provides the energy and processed material driving galactic evolution and setting the stage for subsequent star and planet formation. This collection of papers offers a unique discussion of physical factors that are driven by rapid rotation and whose influence directly impact the evolution and end-of-life state of massive stars. They are presented together to give the reader a perspective that only the ensemble can provide instead of a single paper. We hope that we are successful in our goal of shedding light on the scope and outcome of this important facet of massive star physics.

💡 Research Summary

The editorial “Mass and Angular Momentum Transport of Rapidly Rotating Hot Stars” brings together a suite of papers that explore how rapid rotation and binary interaction shape the evolution of massive stars. It begins by emphasizing the outsized role of massive stars in galactic chemical enrichment, ionizing radiation, and the baryonic cycle, noting that roughly one‑fifth of them rotate faster than 200 km s⁻¹ and that more than 70 % reside in binary systems. The authors argue that the key unknowns are (1) when and how rapid rotation is acquired, (2) what physical changes it induces in the star and its surroundings, and (3) how angular momentum and chemical species are redistributed internally and between companions.

A central theme is the unique diagnostic power of ultraviolet (UV) spectropolarimetry. Because the intrinsic linear polarization of a spherical source is zero, any measured signal directly traces asymmetries such as gravity darkening, stellar oblateness, and circumstellar disks. In the far‑UV, the polarization amplitude can reach 0.1 %–1 % for stars rotating at >80 % of critical velocity, especially when viewed edge‑on. This makes UV polarimetry an ideal tool for measuring near‑critical rotation, disk geometry, and the orientation of the spin axis—measurements that are impossible from the ground due to interstellar polarization (ISP) contamination in the optical.

The collection includes several concrete science cases. Labadie‑Bartz et al. (2025) use BPASS binary population synthesis to map the evolutionary tracks of mass‑donor and mass‑gainer components on the Hertzsprung‑Russell diagram. UV spectropolarimetry can directly detect stripped cores and accretion disks around the companions, confirming the binary‑mass‑transfer origin of many Be stars. Rast et al. (2025a) model how rapid rotation modifies H α equivalent widths and V‑band polarization, showing that the UV polarization signature is the most sensitive probe of inclination and gravity darkening. Rast et al. (2025b) present 3‑D smoothed‑particle hydrodynamics simulations of γ Cas‑type systems, demonstrating that mass transfer onto a white‑dwarf companion creates a quasi‑steady accretion disk that reproduces observed X‑ray luminosities.

Angular momentum transport is examined by Quigley et al. (2025), who adopt viscous decretion‑disk models to quantify how orbital separation and companion mass affect the loss or gain of spin angular momentum in Be binaries. Their results indicate that close binaries can efficiently drain stellar spin, potentially preventing stars from reaching break‑up speed, whereas wider systems allow the primary to retain near‑critical rotation.

The editorial also highlights the “Ohman effect,” a predicted rotation‑induced variation in linear polarization across broadened absorption lines. Harrington et al. (2025b) show that in the far‑UV the effect can produce line‑center polarization changes of up to 1 %, providing a direct spectropolarimetric handle on stellar oblateness, gravity darkening, and surface velocity fields.

Chemical abundance studies are represented by Peters et al. (2026), who measured carbon and nitrogen in eight nearly pole‑on Be stars. While carbon is depleted as expected from CNO processing, nitrogen does not show the anticipated enhancement, suggesting that some nitrogen may be further converted into oxygen or neon during late‑stage mass transfer. This underscores the need for high‑resolution UV spectroscopy of C II, N III, O III, and Ne III lines to fully map the C‑N‑O‑Ne cycle in spun‑up stars.

Finally, Ignace et al. (2025) develop a framework to disentangle intrinsic stellar polarization from ISP by exploiting the wavelength dependence of both components across UV and optical bands. This methodology enables the extraction of pure stellar signals even in heavily reddened environments, thereby improving the reliability of rotation diagnostics.

In summary, the editorial argues that UV spectropolarimetry is the missing observational cornerstone for resolving the intertwined problems of rapid rotation, binary mass transfer, angular‑momentum redistribution, and chemical mixing in massive stars. By assembling these complementary studies, the collection provides a roadmap for future space‑based UV polarimeters, which will be capable of measuring sub‑percent polarization, mapping disk geometry, and probing the internal physics that ultimately governs the fate of the most influential stars in the universe.